If it didn't smell, it's still alive. Once you've smelled a dead snail, you'll never forget it.
A lot of the big chain stores, particularly the ones that sell other animals as well, give terrible advice. Too many of these shops only train their staff in how to sell things.
I have kept nerites for about 15 years. I currently have 6 nerites in my 180 litre tank. I bought the brown/beige striped and red with black spots nerites in July 2011 (yes I do keep notes on when and where I buy fish etc) followed by more in December 2013 and December 2014. They are all still alive. I have never put specific snail food in the tank, but it does grow algae.
You could try a slice of courgette (aka zucchini) last thing at night, so that the fish are asleep and won't eat it all. You would need to weight it down, something like skewering it on an old teaspoon handle would to the job. Or perhaps sushi nori as I know people feed that to otos. Both of these should be removed after 24 hours or they'll go off and make a mess of the tank.
A word of warning about nerites. If you ever have to treat sick fish, remove the nerites. They are extremely easy to kill. I keep mine in my quarantine tank if I need to treat the fish, but a bucket of water with daily water changes would be OK - but cover the bucket with something or the nerites could climb out. Make sure very last trace of medication has gone before putting the snails back.